1. Field of the Invention
The present invention relates to an energy trap type piezoelectric resonator for use in, for example, an oscillator, a discriminator, a trap filter, and other such apparatuses. More particularly, the present invention relates to an energy trap type piezoelectric resonator having a sealed space arranged to allow a vibrating potion to freely vibrate and to generate a third harmonic of a longitudinal vibration in the thickness direction of the resonator.
2. Description of the Related Art
Energy trap type piezoelectric resonators have been used in oscillators, discriminators, trap circuits, and other such apparatuses. In Japanese Examined Patent Publication No. 4-101507, one example of such an energy trap type piezoelectric resonator is disclosed.
In FIG. 8, a prior art energy trap type piezoelectric resonator is shown. In such a conventional piezoelectric resonator 51, the upper and lower sides of a plate-like piezoelectric resonator element 52 are laminated with sealing substrates 53 and 54.
The piezoelectric resonator element 52 is composed of a piezoelectric substrate 55 and vibrating electrodes 56 and 57 provided on both main surfaces of the piezoelectric substrate 55. The vibrating electrodes 56 and 57 are partially formed on the main surfaces of the piezoelectric substrate 55 and are arranged to face each other with the piezoelectric substrate 55 disposed therebetween.
In the piezoelectric resonator 51, the portion in which the vibrating electrodes 56 and 57 face each other constitutes a vibrating portion. Further, sealed spaces 58 and 59 are provided to allow the vibrating portion to vibrate. At areas around the sealed spaces 58 and 59, the piezoelectric resonator element 52 is bonded to the sealing substrates 53 and 54, for example, with adhesive.
In the piezoelectric resonator 51, when the maximum planar dimension of the above vibrating electrodes 56 and 57 is represented by x and the maximum planar dimension of the sealed spaces is L, the vibrating electrodes 56 and 57 and the sealed spaces 58 and 59 are formed so as to satisfy the relation L=(1.5 to 3.0) x , and the resonance characteristic is thought to be stabilized under this condition.
Further, in Japanese Examined Patent Publication No. 7-70941, an energy trap type piezoelectric resonator of this type is disclosed and includes sealed spaces above and below the piezoelectric substrate which are shifted relative to each other. By adjusting the amount of displacement between the upper and lower sealed spaces, the degree of damping of undesired spurious signals can be adjusted.
The piezoelectric resonator 51 described in Japanese Unexamined Patent Publication No. 4-101507 is characterized by the reduced dispersion in resonance characteristics. That is, when the sealed spaces 58 and 59 are arranged to realize the above-specified dimensional ratio, the sealed spaces 58 and 59 can be correctly formed, and consequently, the dispersion in the resonance characteristics is reduced. Accordingly, in Japanese Unexamined Patent Publication No. 4-101507, nothing is mentioned about the influence of other vibration modes causing spurious signals of a resonance characteristic used.
On the other hand, in the piezoelectric resonator described in Japanese Examined Patent Publication No. 7-70941, the relative positions of the upper and lower sealed spaces are shifted, and as a result, it is possible to adjust the degree of damping of spurious signals. However, because it is necessary to move the positions of the upper and lower sealed spaces with respect to each other, the symmetry of the component is destroyed.
Further, in Japanese Unexamined Patent Publication No. 4-101507 and Japanese Examined Patent Publication No. 7-70941, only a piezoelectric resonator adapted to be vibrated in a fundamental wave of longitudinal vibration mode in the thickness direction is disclosed, and there is no mention of any piezoelectric resonator making use of harmonics of longitudinal vibration in the thickness direction thereof.